The invention relates generally to offshore seismic prospecting and, more particularly, to seismic streamers housing groups of hydrophones.
In prospecting for oil and gas deposits beneath the sea floor, survey vessels tow hydrophone cables, or seismic streamers, and seismic sources through the water. Seismic waves emitted periodically by the seismic sources reflect off geologic structures beneath the sea floor. The reflected seismic waves cause pressure changes that are detected by the hydrophones in the streamer. The hydrophones are conventionally divided into groups along the length of the streamer. The outputs of the hydrophones in each group are usually connected electrically in parallel to produce a group response with a higher signal-to-noise ratio than for any single hydrophone response. But some noise sources are not integrated out as well in the group response. Bulge-wave noise in a liquid-filled streamer is caused by the sloshing back and forth of the liquid along the streamer as it is being towed. Flow noise is caused by the shedding of vortices in the turbulent boundary layer surrounding both liquid-filled and solid, or gel-filled, streamers as they are towed through the water. Both bulge-wave noise and flow noise are characterized by oscillatory or periodic pressure variations propagating along the length of the streamer. These pressure fluctuations correlate in time as well as in distance along the streamer. Because the hydrophones in each group are conventionally spaced uniformly along the length of the streamer, the pressure fluctuations are aliased into the group response and lower the signal-to-noise ratio.
For example, the noise gain of a group of eight hydrophones uniformly spaced on 1.6 m intervals is given by the solid plot in FIG. 1. The peaks in the noise gain in the low frequency-region, which includes the bulge-wave and flow noise bands, indicate a relatively high noise correlation for the uniformly spaced hydrophone group.
As another example, the noise gain of a group of fourteen hydrophones 10 arranged along a streamer as shown in FIG. 2, is given in FIG. 3. The hydrophone spacings are mirror images of each other about the group center, with the spacing of the innermost hydrophones Δ=0.875 m and the outermost hydrophones 2Δ=1.75 m. The noise gain is that of a hydrophone group in a conventional MSX™ Active Streamer Section manufactured and sold by Input/Output, Inc. of Stafford, Tex., U.S.A. Although the hydrophones are not uniformly spaced, their positions along the streamer follow a discernible regular pattern as seen in FIG. 2. Because of the hydrophone pattern, the bulge-wave and flow noise gain characteristic of FIG. 3 exhibits undesirable peakiness indicating unwanted noise correlation at the lower frequencies.